1. Field of the Invention
The present invention relates generally to a wireless mobile communication system having a cellular structure (hereinafter referred to as a “cellular wireless mobile communication system”), and in particular, to a system and method for controlling an uplink traffic load.
2. Description of the Related Art
In general, the performance and capacity of a wireless mobile communication system are limited by wireless propagation channel characteristics such as co-channel interference, path loss and multipath fading, occurring between cells or in a cell. Technologies for compensating for the performance and capacity limitations include power control, channel coding, rake reception and diversity antenna technologies.
In a cellular wireless mobile communication system, a plurality of mobile stations (MSs) located in one cell perform wireless communication with a base station (BS) that manages the cell. Therefore, the BS receives uplink signals from each of the MSs. In this case, a signal transmitted by a particular MS may act as an interference component to a signal transmitted by another MS. In addition, if the signal transmitted by the particular MS is high in power, it acts as a high-interference component to the signal transmitted by another MS. Therefore, in the wireless mobile communication system, uplink power control for an MS should necessarily be performed in order for a BS to stably receive signals from MSs.
In the conventional Code Division Multiple Access (CDMA)-based cellular wireless mobile communication system, a BS performs uplink power control for an MS, using a Rise-Over-Thermal (ROT) index. The term “ROT” as used herein refers to a ratio of a value obtained by adding a thermal noise to a sum of signal powers to the thermal noise, and the BS can analyze uplink loading conditions based on the ROT index. The ROT can be represented by Equation (1):
                    ROT        =                                            NS              +              η                        η                    =                                    NS              η                        +            1                                              (        1        )            
In Equation (1), the ROT is defined on the assumption that one of a plurality of possible cells does not suffer interference from its neighbor cells, N MSs are using the same service, and uplink signal power for each of the N MSs is fully controlled by a signal S. In Equation (1), η denotes thermal noise power.
If all MSs located in the cell are power-controlled in a required signal-to-noise ratio (EC/IO)req, the (EC/IO)req can be represented by Equation (2):
                                                                                          (                                                            E                      C                                                              I                      O                                                        )                                req                            =                            ⁢                              S                                                                            (                                              N                        -                        1                                            )                                        ⁢                    S                                    +                  η                                                                                                        ≅                            ⁢                              S                                  NS                  +                  η                                                                                        (        2        )            
In Equation (2), received power S for each of MSs can be expressed as Equation (3):
                                                        S              =                                                                    (                                                                  E                        C                                            /                                              I                        O                                                              )                                    req                                ⁢                                  (                                      NS                    +                    η                                    )                                                                                                        =                                                                    η                    ⁡                                          (                                                                        E                          C                                                /                                                  I                          O                                                                    )                                                        req                                                  1                  -                                                            N                      ⁡                                              (                                                                              E                            C                                                    /                                                      I                            O                                                                          )                                                              req                                                                                                          (        3        )            
Using Equation (3), Equation (1) can be re-written as Equation (4):
                                                        ROT              =                                                NS                  η                                +                1                                                                                        =                                                                    N                    η                                    ·                                                                                    η                        ⁡                                                  (                                                                                    E                              C                                                        /                                                          I                              O                                                                                )                                                                    req                                                              1                      -                                                                        N                          ⁡                                                      (                                                                                          E                                C                                                            /                                                              I                                O                                                                                      )                                                                          req                                                                                            +                1                                                                                        =                              1                                  1                  -                                                            N                      ⁡                                              (                                                                              E                            C                                                    /                                                      I                            O                                                                          )                                                              req                                                                                                          (        4        )            
In accordance with Equation (4), pole capacity indicating the theoretical uplink maximum capacity in the ideal cell environment with no power control and thermal noise can be represented by Equation (5):
                              N          max                =                  1                                    (                                                E                  C                                /                                  I                  O                                            )                        req                                              (        5        )            
Therefore, the ROT of Equation (4) in accordance with Equation (5) can be re-written as Equation (6):
                    ROT        =                  1                      1            -                          N              /                              N                max                                                                        (        6        )            
FIG. 1 is a graph illustrating a variation in ROT with respect to an increase in uplink traffic in a common CDMA communication system. As illustrated in FIG. 1, the ROT is a factor indicating a ratio of the current load to the system pole capacity, and a BS controls an uplink load according to the ROT. After measuring the ROT, the BS broadcasts the measured ROT value to its MSs, and an MS receiving the measured ROT value determines its data rate and transmission power according to the received ROT value.
However, the conventional CDMA-based wireless mobile communication system provides that every MS periodically stops its transmission for a predetermined time in order that a BS may measure the thermal noise power η in an idle state and then measure an ROT in a normal call state. That is, the conventional MSs cannot transmit uplink signals to the BS for a specific time, even though it is for a very short time. In addition, the method of controlling an uplink traffic load using the ROT index cannot be applied to a multi-carrier communication system. Therefore, there is a need for a scheme for controlling an uplink traffic load in the multi-carrier communication system.